A tuning fork crystal oscillator used as a vibrating gyro sensor or the like is manufactured by the steps of cutting a crystal oscillator of a prescribed shape from a crystal wafer, forming electrodes for causing the crystal oscillator to oscillate, and packaging the crystal oscillator with the electrodes formed thereon into a container. Of these steps, the external shape forming step that involves cutting the crystal oscillator from a crystal wafer is an important step since the shape of the crystal oscillator determines the frequency of vibration and greatly affects the performance of the crystal oscillator.
FIG. 1 is a perspective view schematically showing a prior art crystal oscillator.
The external shape of the crystal oscillator 200 is formed by cutting the crystal oscillator from a crystal wafer by etching in the external shape forming step. The crystal oscillator 200 shown here is an oscillator used as a vibrating gyro sensor, and more particularly, a three-pronged tuning fork oscillator having two driving tines 201 and 202 and one detection tine 203 as vibrating tines.
When using the crystal oscillator 200, for example, as a vibrating gyro sensor, flexural vibration in the X axis direction in FIG. 1 is used as driving vibration, and flexural vibration in the Z′ axis direction is used as detection vibration which occurs when an angular velocity is applied. Therefore, when no angular velocity is applied, the vibration in the Z′ axis direction does not occur.
FIG. 2 is a cross-sectional view of the prior art crystal oscillator 200 taken along line A-A′ in FIG. 1, showing the cross section of the driving tine 201, 202. Portions not necessary for the following explanation are not shown in FIG. 2. FIG. 2(a) is a diagram explaining the driving vibration of the driving tine of the crystal oscillator 200, and FIG. 2(b) is a diagram explaining oblique driving vibration occurring in the driving tine of the crystal oscillator 200 and the formation of crystal residues.
FIG. 2(a) shows the ideal driving vibration that should occur when no angular velocity is applied to the crystal oscillator 200, and its vibration direction S1 is parallel to the X axis direction. However, in the crystal oscillator manufactured by the prior art manufacturing method, oblique vibration having a vibration component in the Z′ axis direction is observed (as indicated by the vibration direction S2), as shown in FIG. 2(b), when actually no angular velocity is applied.
This is because, in addition to the driving vibration (the vibration in the X axis direction), an out-of-plane vibration (the vibration in the Z′ axis direction) that should not occur when no angular velocity is applied to the crystal oscillator 200 occurs due to such factors as the processing accuracy of the crystal oscillator and the anisotropy of the crystal.
If the driving tine 201, 202 vibrates obliquely, the distal end of the detection tine 203 vibrates describing a rectilinear motion in the Z′ axis direction or an elliptical motion in the X-Z′ plane. This vibration component in the Z′ axis direction is called the leakage vibration, and because of this leakage vibration, a leakage signal unrelated to the Coriolis output is produced from the detection electrode of the detection tine 203, resulting in the S/N ratio of the gyro sensor dropping or a degrade in temperature characteristics.
Likewise, in the case of tuning fork crystal oscillators used for ordinary applications such as frequency standards, vibrations are produced by utilizing the flexural vibrational motion in the X axis direction, and in this case also, there has been the problem that the leakage vibration containing the Z′ direction component causes the crystal impedance (CI value) to rise, leading to a degradation of the characteristics.
It is believed that manufacturing variations in the cross-sectional shape of the crystal oscillator tines have some bearing on the leakage vibration. In particular, it is believed that variations in the shape of crystal residues generated when manufacturing the crystal oscillator by etching have some bearing on the leakage vibration. That is, the crystal has etching anisotropy, and the etch rate is different in different directions of the crystal. As a result, the side faces of the vibrating tines of the crystal oscillator are not uniformly etched, and residue is left thereon after etching.
The generation of the leakage vibration due to the formation of such residue will be investigated below.
Generally, when investigating the flexure of a beam or the like, the principal axes of its cross section are considered. The principal axes of the cross section are two mutually perpendicular axes, and when a bending force is applied to the beam in the same direction as one principal axis, the beam bends in the same direction as the direction of the applied force. On the other hand, when a bending force is applied in a direction different than the direction of the principal axis, the beam bends in a direction different than the direction in which the force is applied.
In the case of a crystal oscillator, the bending force due to the piezoelectric effect is applied in the X axis direction. Therefore, if one of the principal axes coincides with the X axis, the vibration occurs in the X axis direction, and no leakage vibration is generated. On the other hand, if the principal axis is tilted away from the X axis toward the Z′ axis, since the direction of the bending force does not coincide with the direction of the principal axis, oblique vibration containing a Z′-axis component occurs, resulting in the generation of leakage vibration.
The principal axes are determined by the cross-sectional shape of the beam (vibrating tine). As a simple example, in the case of a cross section having an axis of symmetry, the axis of symmetry and an axis perpendicular to it are the principal axes of the cross section. For example, in the case of a rectangular cross section, the lines that bisect the respective pairs of opposite sides are the principal axes.
If a crystal oscillator free from leakage vibration is to be obtained, it is required that one of its principal axes be parallel to the X axis. Since the principal axes are two mutually perpendicular axes, if the cross section has an axis of symmetry parallel to the X axis or Z′ axis, then there exists a principal axis parallel to the X axis. That is, if the cross-sectional shape is top-bottom or left-right symmetrical, no leakage vibration occurs.
An investigation has been made to see whether a crystal oscillator having such an axis of symmetry can be obtained when the oscillator is manufactured as in the earlier described example. When the crystal oscillator is manufactured using wet etching, residue is invariably left on the side faces of the vibrating tine. The principal axes of its cross section are therefore determined depending on how the residue is formed. When considering the principal axes of the cross section of the crystal oscillator, first it is necessary to examine how the residue is formed. Since residue shape varies depending on the etching time and etching conditions, it is not possible to generalize, but the process of formation is roughly the same; therefore, the process of residue formation will be described below based on the results observed from the experiment conducted by the present inventor.
FIG. 3 is an enlarged cross-sectional view, taken along line A-A′ in FIG. 1, schematically showing the vibrating tine of the crystal oscillator 200 to illustrate one example of how the residues are formed on the vibrating tine. For simplicity of explanation, only one driving tine 201 is shown here, and the side face on the −X side of the crystallographic axis of the crystal is denoted as the first side face and the side face on the +X side as the second side face.
FIG. 3(a) shows the case where the etching time is relatively short. In this case, the residue is formed on the second side face, forming an angle of about 2° with the Z′ axis in regions (shallow regions) near the principal faces, i.e., the upper and lower surfaces 201a and 201b, of the oscillator and an angle of about 22° in regions (deeper regions) farther away from them.
Though the depth from the upper and lower surfaces 201a and 201b varies depending on the etching time, the process is essentially the same for both the upper and lower surfaces 201a and 201b. 
FIG. 3(b) shows the case where the etching time is relatively long. In this case, the regions forming the angle of about 22° are etched away, and only the residue regions forming the angle of about 2° are left unetched.
In either case, the residue formed on the first side face is very small, but when closely observed, the residue is certainly formed, as shown in FIGS. 3(a) and 3(b). In this case, the residue is formed, forming an angle of about 1° with the Z′ axis. The shape of the residue on the first side face is relatively unaffected by the etching time. That is, the etching starts from the edges of the respective etching masks 250a and 250b, and proceeds on both the upper and lower surfaces independently of each other until the wafer is etched through.
Since the residues are formed as a result of etching as described above, the following can be pointed out when the crystal oscillator is manufactured by the method that performs etching from both the upper and lower surfaces of the crystal wafer. First, FIGS. 3(a) and 3(b) each show the case where the etching mask 250a formed on the upper surface of the crystal wafer and the etching mask 250b on the lower surface are in perfect registration. In this case, whether the etching time be short or long, the cross section of the vibrating tine 201 after etching is top-bottom symmetrical about an axis of symmetry substantially parallel to the X axis, and has a principal axis 210 substantially parallel to the X axis, as illustrated. In this case, leakage vibration does not easily occur because the direction of the bending force and the direction of the principal axis 210 both substantially coincide with the X axis.
FIG. 4 shows one example of a cross-sectional view of the driving tine 201 which was formed when the etching masks 250a and 250b were formed one displaced from the other in the X axis direction.
As shown, the cross-sectional shape of the driving tine 201 becomes top-bottom asymmetrical, and does not have an axis of symmetry parallel to the X axis, nor does it have an axis of symmetry parallel to the Z′ axis.
In this case, the principal axis 211 is not parallel to the X axis, but is displaced by an angle θ1. As a result, since the direction of the bending force and the direction of the principal axis are different, oblique vibration occurs, resulting in the generation of leakage vibration. There is a document that analyzes the relationship between the oblique vibration and the principal axes of such a cross section (for example, refer to non-patent document 1).
As can be seen from FIG. 4, there is correlation between the amount of displacement, e, of the etching masks 250a and 250b and the angle of displacement, θ1, of the principal axis 211 relative to the X axis. As the amount of displacement, e, increases, the angle of displacement, θ1, also increases, increasing the leakage vibration.
In one known method employed to form the external shape of a crystal oscillator, an etching mask is patterned only on one surface of a crystal wafer, the other surface is completely covered with a corrosion resistant metal film, and etching is performed from the one surface; in another known method, the etching mask pattern formed on the lower surface is made wider than the etching mask pattern on the upper surface, and etching is performed using the upper etching mask pattern as the reference pattern.
FIG. 5 is a cross-sectional view of a driving tine, showing one example of the etching performed using the upper etching mask pattern as the reference pattern.
The driving tine 221 shown here is formed by using the etching mask 251a on the upper surface 221a, which is set as the reference pattern, and the etching mask 251b on the lower surface 221b, which is formed wider than the upper surface. In this case, even if the etching masks 251a and 251b are somewhat displaced relative to each other, the cross-sectional shape is relatively unaffected. However, as described earlier, due to the etching anisotropy of the crystal, a residue forming an angle of about 1° with the Z′ axis is formed on the first side face, and a residue forming an angle of about 2° with the Z′ axis is formed on the second side face. As a result, the cross-sectional shape of the driving tine 221 is top-bottom asymmetrical as shown, and the principal axis 212 is not parallel to the X axis, but is displaced by an angle θ2, resulting in the generation of leakage vibration.
As described above, crystal oscillators used for such applications as vibrating gyro sensors have had the problem that leakage vibration occurs due to etching mask formation errors, resulting in a degradation of sensor detection accuracy, etc.
To solve the above problem, if the accuracy of etching mask formation is increased, the leakage vibration can be suppressed to a certain degree, but this not only increases the cost, but there is a limit to the degree to which the accuracy can be increased. Furthermore, since the residues cannot be completely eliminated, it has been difficult to suppress the leakage vibration.
In view of the above, a method has been proposed that further processes the vibrating tines after forming the external shape of the crystal oscillator. For example, a crystal oscillator characteristic adjusting method is disclosed that involves grinding the edges of the vibrating tines of the crystal oscillator on a sliding tape and adjusting the balance of the vibrating tines, thereby aiming to suppress the occurrence of leakage vibration (for example, refer to patent document 1).
FIG. 6 is a schematic diagram for explaining the crystal oscillator characteristic adjusting method disclosed in patent document 1.
While applying a constant load to a tape 301 by the weight of a tension roller 300, the tape 301 is placed in contact with an edge of a vibrating tine 310 of the crystal oscillator mounted on a base 311. It is assumed here that the vibrating tine of the crystal oscillator has a cross section such as shown in FIG. 2. In this condition, a driving roller 302 is rotated back and forth, and the weight balance of the vibrating tine 310 is adjusted by forming a ground portion 310a on the vibrating tine 310.
According to the prior art disclosed in patent document 1, the tape 301 can be moved in sliding fashion while pressing it against the edge of the vibrating tine 310 by applying a constant force using the tension roller 300. Finally, a tape take-up reel 303 is rotated by a predetermined amount to take up the tape 301, and at the same time, a tape supply reel 304 is rotated by a predetermined amount to feed the tape 301. With this operation, the portion of the tape 301 that contacts the vibrating tine can be changed to a new portion. According to the characteristic adjusting method that uses such a tape, since the weight balance can be precisely adjusted, the angular velocity can be accurately detected, and since the tension roller 300 serves to prevent an excessive eternal force from being applied to the oscillator, the method has the potential of being able to prevent the breakage of the oscillator.
Instead of using such a tape, the vibrating tines of the crystal oscillator may be ground by etching (for example, refer to patent document 2).
Patent document 2 discloses an oscillator temperature characteristic adjusting method that, after forming the external shape of the piezoelectric oscillator, adjusts the frequency-temperature characteristics of the oscillator by adjusting the plate thickness by performing etching using a metal film such as an electrode film as a mask. According to this prior art method disclosed in patent document 2, since the temperature characteristics can be adjusted by performing re-etching after forming the oscillator, the method has the potential of being able to efficiently manufacture oscillators having excellent frequency-temperature characteristics.    Patent document 1: Japanese Unexamined Patent Publication No. 2002-243451 (Page 7, FIG. 9)    Patent document 2: Japanese Unexamined Patent Publication No. S54-53889 (Page 3, FIG. 5)    Non-patent document 1: Motohiro FUJIYOSHI and five others, “Modeling and Vibration Analysis of Quartz Gyro Sensor,” IEICE Transactions, C Vol. J87-C, No. 9, pp. 712-719